Which of the following best explains why ammonia (NH_3) is not a planar molecule?

• A. Ammonia forms double bonds that force the molecule out of planarity.
• B. The presence of a lone pair on the nitrogen atom causes a trigonal pyramidal geometry.
• C. Hydrogen atoms in ammonia repel each other, resulting in a linear structure.
• D. Nitrogen in ammonia is sp^2 hybridized, leading to a bent shape.

Answer: (B)

Verified step by step guidance

1. Identify the central atom in ammonia (NH_3), which is nitrogen (N), and note that it has three hydrogen atoms bonded to it along with one lone pair of electrons.
2. Recall that the molecular geometry depends on both bonded atoms and lone pairs; lone pairs occupy space and influence the shape by repelling bonded atoms.
3. Understand that nitrogen in ammonia is sp^3 hybridized, meaning it has four regions of electron density (three bonds and one lone pair), which typically leads to a tetrahedral electron geometry.
4. Recognize that the presence of the lone pair on nitrogen pushes the three hydrogen atoms downward, resulting in a trigonal pyramidal molecular shape rather than a planar one.
5. Conclude that the lone pair's repulsion is the key reason ammonia is not planar, ruling out explanations involving double bonds, linear shape, or sp^2 hybridization.